1. Field of Invention
The present invention relates to an Fexe2x80x94Ni alloy material used for a shadow mask subjected to fine etching. More particularly, the present invention relates to an Fexe2x80x94Ni alloy material used for a shadow mask, which enables through-holes for passing an electron beam to be formed by etching, having improved uniformity of diameter. The present invention relates to the Fexe2x80x94Ni alloy material used for a shadow mask, having etched through-holes with improved uniformity of diameter.
2. Description of Related Art
Heretofore, mild steel has been generally used for the shadow mask of a CRT. However, when the CRT is continuously operated, the temperature of the shadow mask rises due to the radiation of an electron beam. As a result of thermal expansion of the shadow mask, coincidence of the fluorescent material and the irradiation point of an electron beam is not maintained, thereby resulting in color deviation. When the color image tube is operated, one third or less of the electron beam passes through the apertures, while the rest of the electron beam is irradiated and impinged on the shadow mask, elevating its temperature.
Accordingly, Fexe2x80x94Ni alloy having a small coefficient of thermal expansion, referred to as xe2x80x9c36 Alloyxe2x80x9d has been used in recent years from the viewpoint of color deviation in the field of a shadow mask used for a CRT.
In the production process of the Fexe2x80x94Ni alloy shadow-mask material, a predetermined Fexe2x80x94Ni alloy is vacuum-melted, for example, in a VIM furnace or ladle-refined in LF, and then cast into an ingot. The alloy is forged and then hot-rolled into a slab. The oxide scale on the surface of the slab is removed. Cold-rolling and annealing (recrystallizing annealing) are repeated. After the final annealing, the final cold-rolling is carried out to finish the sheet to a predetermined thickness, i.e., 0.3 mm or less. Thereafter, slitting is carried out to a predetermined width. After degreasing of the so-produced material for a shadow mask, photoresist is applied on both surfaces of the material. A pattern is printed on the photoresist and then developed. The etching is then carried out with an etchant. The material is then cut into separate flat masks. The flat masks are annealed in non-oxidizing atmosphere so as to impart press formability. In the case of the pre-annealing method, the annealing is applied to the finally rolled material prior to the etching. Pressing into a spherically shape is carried out. Finally, the spherically-shaped mask is degreased and is then subjected to blackening treatment in steam or combustion-gas atmosphere to form a black oxide film on the surface. The shadow mask is produced as above.
The finally cold-rolled material, which is or has been subjected to etching for forming the through-holes for passing an electron beam, is herein collectively referred to as the material used for a shadow mask. The flat mask is, therefore, included in the material used for a shadow mask. The material, on which the through-holes have been formed, but which is not yet press-formed, is also included in the material used for a shadow mask.
The through-holes for passing an electron beam are formed in the shadow mask by means of the well known etching usually using a ferric chloride aqueous solution. Before the etching, the well known photolithography technique is applied in such a manner that the photoresist mask is delineated to form a number of apertures in the circular form having, for example, 80 xcexcm of diameter on one of the surfaces of the alloy strip and to form a number of apertures in the circular form having, for example, 180 xcexcm of diameter on the coincident positions of the other surface of the alloy strip. The aqueous-solution of ferric chloride in the form of spray is blown onto the alloy strip.
The shadow mask, on which minute apertures are densely arranged, is obtained by the etching mentioned above. Local variation in etching conditions results in deviation of the diameter of apertures. When such variation becomes excessive, color shift occurs in a Braun tube mounting such shadow mask. Such mask is, therefore, unacceptable. In the production of shadow masks, the yield has heretofore been lowered and hence the cost has been increased due to variation of the aperture diameter.
Various considerations have heretofore been made to improve the etching formability of through-holes. Japanese Unexamined Patent Publication No. 05-311357 is related to improvement of the material and proposes to control the texture degree of the {100} plane on the rolling plane to less than 35% and hence randomize the crystal orientation. Japanese Unexamined Patent Publication No. 5-311358 describes to limit the total length of inclusions in the rolling direction per unit area of the parallel cross-section to the rolling direction. In addition, Japanese Unexamined Patent Publication No. 7-207415 describes that the etching formability of through-holes is improved by means of limiting the Mn and S concentrations as well as the Si and C concentrations, and also by controlling the cleanliness of the oxide-based inclusions of the cross section of the material.
The present inventors carried out intensive research and discovered that the local etching failure of through-holes for passing the electron beam described below cannot be prevented by means of controlling the texture and limiting the inclusions. Excessive etching of the apertures as compared with the neighboring apertures may occur, resulting in etching failure. As a result of the failure in local etching, the diameter of through-holes for passing an electron beam varies. This etching failure discovered by an inventor is a phenomenon that, when the shadow mask, in which the through-holes for passing an electron beam has been formed by means of etching, is observed in such a manner that an observer sees the light through the mask, the vicinity of apertures appears light and shines. FIG. 1 is an enlarged drawing of a normal aperture, while FIG. 2 is an enlarged view of an abnormal aperture. When the wall of the normal and abnormal apertures are observed, the inclination angle of the wall is seen smaller in the abnormal aperture (FIG. 4) than that of the normal aperture (FIG. 3). Because of very local etching failure around the periphery of an abnormal aperture, the aperture diameter tends to be greater than the target value.
It is an object of the present invention to provide an Fexe2x80x94Ni alloy material, which has through-holes formed by etching, without variation of the diameter which is attributable to local etching failure of the Fexe2x80x94Ni alloy material during etching to form through-holes for passing an electron beam.
The present inventors carried out intensive research to attain the object mentioned above from a novel point of view not found in the prior art, particularly the reasons for the local corrosion anomaly mentioned above. As a result, it is found that fine precipitates and inclusions present in the Fexe2x80x94Ni alloy material exert great influence upon the etching of through-holes for passing the electron beam. Such local etching failure and hence the diameter variation of etched apertures are difficult occur in the Fexe2x80x94Ni alloy material, in which a large number of fine precipitates and inclusions are present in the material as a whole. It was found that, when the precipitates and inclusions from 0.01 xcexcm to 5 xcexcm in size are present on the surface of material at a frequency of 2000 or more per mm2, the precipitates and inclusions are effective for suppressing the above mentioned variation.
The components of the precipitates and inclusions were identified. The identified precipitates and inclusions are nitrides such as BN, TiN, AlN and the like, oxides such as MnO, MgO, CaO, TiO, Al2O3, SiO2 and the like, sulfides such as MnS, CaS, MgS2 and the like, and carbides such as TiC, SiC and the like. When a sample is immersed in the acidic solution such as dilute hydrochloric acid, dilute sulfuric acid solution, and the sample is anodically dissolved in the acidic solution at a potential in an active dissolving region, the particles of precipitates and inclusions appear in the form of pits (pitting corrosion). The frequency of the particles of precipitates and inclusions can be evaluated as the pit density in number per mm2.
It is not precisely elucidated how the minute inclusions or precipitates can suppress variation of the diameter of etching apertures. It can be postulated as follows.
The Fexe2x80x94Ni alloy, to which the present invention relates, is usually etched by means of a ferric chloride-containing aqueous solution to form the through-holes for passing an electron beam. During the etching, resist film is applied on the material, where no apertures are to be formed, while the portions of the material, where the apertures are to be formed, are brought into the ferric chloride aqueous solution. When the minute inclusions or precipitates (hereinafter collectively referred to as the inclusions, unless otherwise specified) are present on the latter portions, the inclusions behave as the origin of corrosion, thereby promoting corrosion of the matrix. If no inclusions are present on the aperture portions at all, all of these portions undergo identical etching so that the diameter of apertures does not vary. However, it is difficult in the actual industrial production to provide a completely inclusion-free material. Inclusions are thus present on several aperture portions in a certain probability. The etching rate in the first aperture portions, where the origins of corrosion are present, is higher than that in the second aperture portions in the neighborhood of the first aperture portions, where no origins of corrosion are present. The aperture-diameter of the first aperture portions is greater than that of the second aperture portions. The first aperture portions become electrochemically anode, while the second aperture portions become electrochemically cathode. The difference in the etching rate between the first and second aperture portions is further increased. At the completion of etching, the difference between the aperture diameters is, therefore, great.
On the other hand, when fine inclusions are present in the material at a certain frequency, the inclusions can be present uniformly in all aperture portions. The diameter of apertures then does not vary.
As a result of the elucidation mentioned above, it can be said as follows. When the inclusions, which are the origin of corrosion, are present at a frequency less than a certain level, the uniform distribution of inclusions on the entire material is lost. There are following aperture portions. In most of aperture portions, the inclusions are present and are related to the corrosion. The degree of relation is in average in these aperture portions. The inclusions are not related to corrosion in other aperture portions. The inclusions are related to corrosion in a degree higher than the average one in still other aperture portions. The relation of inclusions and corrosion in all of these aperture portions is different from one another. The corrosion rate in these aperture portions is different from one another. The wall, profile and diameter of apertures are influenced by the different etching rate. The local etching failure occurs on the wall, profile and diameter of apertures formed by etching under different rates. The local etching failure and hence the diameter variation of the etched through-hole can be observed under an electron microscope. The presence of inclusions can be confirmed as the pits mentioned above. The inclusions and the pits are present in ratio of almost 1: 1.
As is described hereinabove, more than certain numbers of fine inclusions are positively introduced in the matrix of Fexe2x80x94Ni alloy in the present invention. This measure is contrary to the conventional concept. The local etching failure is eliminated by such inclusions and the variation of the aperturexe2x80x94diameter is eliminated or lessened.
In accordance with the objects of the present invention, there is provided a material used for a shadow mask having improved uniformity in the diameter of apertures formed when etching the through-holes for passing an electron beam, wherein said material is an Fexe2x80x94Ni alloy consisting of, by mass percentage (%), (hereinafter simply referred to as the mass %) from 34 to 38% of Ni, not more than 0.5% of Mn, and if necessary, from 5 to 40 ppm of B and from 5 to 40 ppm of N, the balance being Fe and unavoidable and incidental impurities with the proviso of 0.10% or less of C, 0.30% or less of Si, 0.30% or less of Al, 0.005% or less of S, and 0.005% or less of P, characterized in that 2000 or more of precipitates and inclusions from 0.01 xcexcm to 5 xcexcm in diameter are varied on the surface of said material per mm2 of said surface.
The diameter of inclusions is the diameter of the smallest circle, in which an inclusion is included.
There is also provided a post-etched material. That is, material used for a shadow mask having through-holes for passing an electron beam formed by etching, with improved uniformity in the diameter of apertures formed, consists of an Fexe2x80x94Ni alloy consisting of, by mass percentage (%), from 34 to 38% of Ni, not more than 0.5% of Mn, and, if necessary, from 5 to 40 ppm of B and from 5 to 40 ppm of N, the balance being Fe and unavoidable and incidental impurities with the proviso of 0.10% or less of C, 0.30% or less of Si, 0.30% or less of Al, 0.005% or less of S, and 0.005% or less of P, characterized in that 2000 or more of precipitates and inclusions from 0.01 xcexcm to 5 xcexcm in diameter are varied on the surface of said material per mm2 of said surface, except for the portions where said through-holes are formed.
In the Fexe2x80x94Ni alloy material according to the present invention, the Ni content is limited in a range of from 34 to 38%. When the Ni content falls outside this range, the coefficient of thermal expansion becomes so great that the Fexe2x80x94Ni alloy cannot be used as a shadow mask. The detrimental effects of S, which impairs the hot-workability, is eliminated by Mn added to iron alloy. However, when the Mn content exceeds 0.5%, the material is excessively hardened and the workability is impaired. The highest content of Mn content is, therefore, limited to 0.5%.
The Fexe2x80x94Ni alloy material contains as impurities or incidental impurities C, Si, Al and P. The upper limits of C, Si, Al and P are limited to 0.10%, 0.30%, 0.30% and 0.005%, respectively. When the concentrations of these elements are more than a certain level, the etching formability of through-holes is so impaired that the material cannot be used for a shadow mask. When the S content is more than 0.005%, the hot workability of material is seriously impaired. The highest content of S is, therefore, limited to 0.005%.
In addition, from 5 to 40 ppm of B and from 5 to 40 ppm of N are contained for the purpose of introducing fine BN particles.